1. Field of the Invention
This invention relates generally to gas turbine engine fuel systems and, more particularly, to an improvement in systems for conveying metered quantities of powdered fuel, such as coal, to a gas turbine engine combustor.
2. Description of the Prior Art
In gas turbine engines operated on solid fuels, such as powdered coal, the fuel is usually stored at atmospheric pressure and metered to a high pressure delivery circuit for movement to the engine's combustor against the compressor discharge pressure existing in the latter. In U.S. Pat. No. 4,342,428, issued Aug. 3, 1982 to Thomas P. Kosek and assigned to the assignee of this invention, means are disclosed for metering powdered coal from low pressure storage into a high pressure stream of air flowing from an air pump to the engine's combustor. Reference may also be made to U.S. Pat. No. 4,296,888 issued Oct. 27, 1981 to Kosek and Steinhilper and assigned to the assignee of this invention, for a description of a nozzle for injecting powdered coal into a combustor. In order for the airstream to flow from the air pump to the combustor, the pump discharge pressure must exceed compressor discharge pressure in the combustor. Most desirably, the pump discharge pressure should exceed compressor discharge pressure by an amount sufficient to effect efficient clean-out of fuel from the metering device and transport to the combustor but not by so much as to reduce combustion efficiency. In U.S. Pat. No. 4,392,347, issued July 12, 1983 to Everett Shows and assigned to the assignee of this invention, a system is disclosed wherein the difference between air pump discharge pressure and compressor discharge pressure is maintained constant throughout the working range of the engine. A gas turbine engine fuel system according to this invention represents an improvement over this and other known systems in that it maintains a constant ratio between air pump discharge pressure and compressor discharge pressure throughout the working range of the engine.